The present invention relates to a system and method for removing charged species present as contaminants in electrically conductive material. More specifically, the present invention provides a system and method whereby soluble salts are electrochemically/electrokinetically separated from an electrically conductive material contaminated therewith, thus enabling the safe disposal and/or recycling of the residual electrically conductive material.
When electrically conductive materials are utilized in harsh chemical environments, ionic species present in the environment often become incorporated in the conductive material. This is especially so when the conductive material is simultaneously exposed to elevated temperatures approaching 1000xc2x0 C. For example, the Hall process for converting alumina to aluminum employs an electrolytic cell or xe2x80x9cpotxe2x80x9d constructed of a steel shell lined with a non-conductive insulating brick with an inner liner of conductive carbon. The cell holds a molten cryolite-alumina electrolyte. The carbon bottom of the electrolytic cell is covered by a pad of molten aluminum and functions as a cathode. The anodes are carbon structures suspended in the electrolyte. During electrolysis, alumina is reduced to aluminum at temperatures in the range of 930xc2x0 C.-980xc2x0 C. The aluminum thus produced is deposited on the metal pad and oxygen, which is produced at the anode, reacts with the carbon to form carbon dioxide. Over a period of several years, the pot liner becomes degraded and breaks down. This is due, in part, to migration of salts from the electrolyte bath into the pot liner, which ultimately causes it to fail. The failed pot liner material is known in the industry as spent pot liner (SPL).
SPL material contains a relatively small but significant amount of cyanide (ranging from about 0.03-0.6 weight percent), a few heavy metals and substantial amounts of sodium and fluoride salts. Fluorides may comprise up to 20 weight percent of SPL material. The cyanide results from the thermal processing used during aluminum production and the fluoride is a component of commonly used aluminum flux compounds. Recent industry estimates indicate that more than 230,000 tons of SPL material is produced each year. Because of its cyanide content and high concentration of leachable fluoride, SPL material has been listed as a hazardous waste by U.S. federal and state environmental authorities. Current federal regulations and those of many states require that SPL material be treated to remove toxic cyanide and leachable fluoride constituents before it can be placed in a landfill disposal site.
Removal of the cyanide and fluoride constituents of SPL material would improve the value of the material, changing it from a hazardous material to a useful product of commerce. For example, decontaminated SPL material may serve as a valuable carbon source, which could be comminuted and burned in a commercial boiler. Moreover, SPL material contains other valuable, recoverable material that can be recycled, including, without limitation, cryolite and aluminum.
Various approaches have been proposed for rendering SPL material non-hazardous. These have included chemical treatment, pyrolysis, oxidation and agglomeration in a fluidized bed, combustion, roasting, as well as combinations of the foregoing. Many of these processes are described in U.S. Pat. Nos. 4,927,459 and 5,683,663. For various reasons, these processes have not been shown to be entirely satisfactory, either due to the capital expense involved or the inability to meet the increasingly strict standards imposed by U.S. federal and state environmental authorities for landfill disposal.
More recently, a vitrification process has emerged as a commercially viable solution to the SPL disposal problem. However, this approach does not allow for the recovery or recycling of valuable constituents of SPL materials.
From the foregoing background, it will be appreciated that a need exists for a process for decontaminating SPL material which is effective in meeting federal and local environmental standards for disposal and enables recycling of economically valuable constituents of such materials, if desired.
In accordance with the present invention, a system and method are provided for removing charged species from contaminated electrically conductive materials. Although the present invention is particularly adapted to the removal of cyanide and fluoride salts from SPL materials, it can be implemented efficiently and cost effectively for treatment of a wide range of contaminated electrically conductive materials.
The system of the present invention comprises an anode compartment including an anode, a cathode compartment, including a cathode and a treatment zone which is in fluid communication with the anode and the cathode compartments for containing the contaminated electrically conductive material undergoing treatment. The anode compartment, cathode compartment and treatment zone are arranged so that a voltage gradient applied between the anode and the cathode produces an electrical current that flows through the material in the treatment zone. This current induces water and charged species to migrate from the material undergoing treatment toward either the anode compartment or the cathode compartment, depending upon whether the charge of the contaminant is positive or negative.
The system of the present invention may, if desired, include a pH controller for monitoring and adjusting the pH of the treated material.
The method of the present invention comprises providing an electrochemical/electrokinetic apparatus in which a sample of the contaminated electrically conductive material to be treated is deposited in the treatment zone of the above-described apparatus. A voltage gradient is then applied between the anode and the cathode of the apparatus. The voltage gradient causes an electrical current to flow between the cathode and the anode through the material to be treated. The current induces water and charged species within the material undergoing treatment to migrate either towards the anode or towards the cathode, depending upon the predominant surface charge of the material undergoing treatment and whether the ionic species constituting the contaminant are negatively or positively charged. The movement of water and charged species causes the charged species to be concentrated in certain areas of the apparatus where they can then be isolated.
The process of the invention can be used to both extract hazardous contaminants from electrically conductive materials, such as SPL materials, and to destroy them.
The residue resulting from the above-described process may be further treated to recover constituents having economic value, such as carbon, cryolite and aluminum, e.g., in the case of SPL materials. The recovered constituents can readily be recycled into usable products.
The foregoing process affords technical, economic and regulatory advantages as compared with existing and emerging processes proposed for treating SPL materials.